The study, published in the journal Science (Aug. 20, 1999), found that none of the patients had been infected with PERV, even though 23 of them still had pig cells circulating in their blood. The company’s press release concluded that the study “demonstrated that pig tissue can survive in the human body for long periods with no ill effects.” Writing in the same issue of Science, Robin A. Weiss, of the Windeyer Institute of Medical Sciences at University College in London, says the study offers “some reassurance on safety” but adds that questions remain, including those about other possible pig viruses.

“The possibility remains that, say, one among 1,000 xenograft recipients may become infected by PERV,” Weiss notes. “The concern then will be the potential for onward transmission from the rare, infected transplant recipient to his or her contacts. Lest we dismiss this notion as ridiculous, we should bear in mind that HIV-1 began as zoonosis (animal-to-human disease), probably from chimpanzees, and that the worldwide pandemic of HIV-1 subgroups may be attributable to a single cross-species event.”

In the meantime, Novartis Pharma AG, the Swiss pharmaceutical giant and Imutran’s parent company, has been testing its pig organs in baboons. So has its main competitor, Nextran, a Princeton, New Jersey-based subsidiary of medical product maker Baxter International. After the PERV study appeared in Science, Novartis officials said they need to “refine” their anti-rejection drug combination before they can proceed to human studies. They’ll also need approval, here in the United States, from the Food and Drug Administration.

But the pressure is on. Novartis makes cyclosporine, the drug nearly every transplant patient takes to fight rejection. If they succeed with xenotransplantation, demand for the drug will skyrocket. Weiss and Purdy agree that the potential benefits of successful xenotransplantation are huge for individuals suffering from organ failure. For the public, however, the risks are as high as they are speculative.

“Although it now seems unlikely that each of the long series of steps required for the worst-case scenarios would occur, we do not know this, nor can we forecast the consequences if just a few of those steps occurred,” Purdy writes. “Because xenotransplantation could extend life if rejection problems are overcome, it is hoped that these fears will prove groundless. However, because of the possibility of risk to third parties, the scientific community must show why it is reasonable to proceed.”